1. Field of the Invention
The present invention relates to a conductive paste for a solar cell, and more particularly, to an electrically conductive paste used to form grid electrodes for Si solar cell.
2. Technical Background
Silver paste is widely used for the electrode paste used in silicon solar cells, since electrode pastes for solar cells are required to have low electrical resistance to facilitate improved efficiency.
In the case of silicon solar cells in which electrodes are formed on both sides, the light receiving side paste usually contains, as basic components, electrically conductive particles in the form of Ag, binder, glass frit and a solvent (see, for example, Japanese Patent Application Laid-open No. 2006-295197). Silver is generally used as the metal powder for grid electrodes in solar cells. In Japanese Patent Application Laid-open No. 2006-295197, examples of electrically conductive particles include metal particles selected from the group consisting of Cu, Au, Ag, Pd, Pt, alloys of Cu, Au, Ag, Pd and Pt, and mixtures thereof.
A typical method for producing solar cell electrodes is to a) print a solar cell paste onto certain locations on a substrate, and b) fire the solar cell paste in a firing furnace. When a screen printing process is used, electrodes for solar cells in which the substrate is crystalline silicon are often fired at a peak temperatures of 750 to 800° C. using a belt type IR furnace.
It would be desirable, in the interest of improving solar cell production efficiency, to fire several solar cells at the same time, in a large-scale firing furnace. However, a certain degree of variation in temperature can occur in firing furnaces, which can adversely affect the solar cell conversion efficiency. When electrode paste is used for ceramic substrates, degree of variation in firing temperature only affects on line resistance slightly. In solar cells, however, the variation is an element directly related to conversion efficiency, which is a key factor related to the quality of solar cells, and even a small reduction would be welcome.
When currently available electrode paste is used, there is about 30° C. of variance in the firing conditions that are considered suitable in terms of conversion efficiency. Outside of this temperature range, the conversion efficiency decreases precipitously. One reason that the conversion efficiency of solar cells is dependent on the firing temperature range is that a good electrical connection between the electrodes and solar cell substrate occurs within a particular temperature range and such a good connection is not obtained when firing takes place outside the suitable firing temperature range. When large-scale firing furnaces are used, requiring a narrow temperature range leads to lower yields. A more expanded range of suitable firing temperatures would be desirable both for large scale firing arrangements and where small-scale firing furnaces are used.